Belief in causality is deeply held by many philosophers and scientists. Many say it is the basis for all thought and knowledge of the external world. The idea that every event has a cause leads to the view of causal closure or causal determinism. The simplest form of physical determinism is the Laplacian view that given the positions and velocities of all the fundamental particles in the world, together with the laws of nature, that there is only one possible future.

Reductionism is the view that material particles and the physical forces between them, which are supposed to be mathematically analytical, can explain all that happens in the world. Chemistry is thought to be reducible to physics, biology reducible to chemistry, psychology (via neuroscience) reducible to biology, and mind/brain (or cognitive science) reducible to psychology. All these causal relations are called "bottom-up."

The finest details of brain events are thought by some to be a consequence of motions of the material particles that comprise the brain. Reductionism implies that mind is an epiphenomenon, or worse, just an illusion. The reductionist idea that everything is the consequence of "bottom up" physical causes is often called eliminative materialism.

By contrast, downward causation is a kind of holism that denies reductionism. "Wholes" can enforce constraints on their "parts" to make them move in ways that may be unpredictable, even given the complete information about the parts (ultimately the atoms and molecules) along with the complete information about the state of the universe outside those parts.

Downward causation is closely related to the concepts of emergence, self-organization, and supervenience. It has become very popular in the study of complex physical systems which exhibit a kind of self-organization and emergence of visible structures when the systems are far from equilibrium conditions.

Most modern discussions of emergence and self-organization in hierarchical systems start with the early 1950's work of Ilya Prigogine on dissipative structures, physical and chemical systems that are far from equilibrium, through which there is a steady flow of matter and energy.

Despite the normal tendency to chaos (the second law of thermodynamics and increasing entropy), these dissipative systems develop relatively stable visible structures, such as Bénard convection cells and Turing autocatalytic reactions that show space-dependent, steady-state processes stable against perturbations. These visible structures reduce the entropy locally. They are "information structures" of a very simple kind.

Prigogine's discovery of such "order out of chaos" in physical systems is widely cited as evidence of emergent properties in complex adaptive systems. It lies at the heart of modern complexity theory and chaos theory. But these complex and chaotic information structures are "dumb." They do not contain the internal information processing that marks the emergence of life and mind. These two higher-level emergent information structures are "smart," by comparison. When they exert downward causation, it is extraordinarily fine.

The idea that emergent structures can exert a sort of "holistic" downward causal control on their molecular components was first articulated by Roger Sperry in 1965. Sperry cites a wheel rolling downhill as an example of downward causal control. The atoms and molecules are caught up and overpowered by the higher properties of the whole. Although this is a very gross kind of control over the components, Sperry says that he "worked the new mind-brain ideas into a discussion of holist-reductionist issues, emergent downward control. and ‘nothing but’ fallacies in human value systems, in a broad refutation of the then prevalent ’mechanistic, materialistic, behavioristic, fatalistic, reductionistic view of the 'nature of mind and psyche’."

In 1974 Donald Campbell coined the phrase "downward causation" and he is widely cited in the current literature as the main source of the idea.

Some biologists (e.g., Ernst Mayr) have argued that biology is not reducible to physics and chemistry, although it is completely consistent with the laws of physics. Even the apparent violation of the second law of thermodynamics has been explained because living beings are open systems exchanging matter, energy, and especially information with their environment.

Information is neither matter nor energy, but it needs matter for its embodiment and energy for its communication.

A living being is a form through which passes a flow of matter and energy (with low or "negative" entropy, the physical equivalent of information). Genetic information is used to build the information-rich matter into an overall information structure that contains a very large number of hierarchically organized information structures. Emergent higher levels exert downward causation on the contents of the lower levels.

The problem of mental causation is a specific case of downward causal control that is central to the philosophy of mind.

The idea that minds have powers "over and above" the known physical, chemical, and biological laws is sometimes called "mentalism." It is related to the idea of "vitalism," that biology might involve new laws that cannot be reduced to "nothing but" the laws of physics.

Examples of Downward Causation

When the earth turns, or revolves about the sun, or travels with the sun through the spiral arms of our galaxy, everything on earth is carried along with it.

When a wheel rolls, its component maolecules roll along with it (Roger Sperry's example)

When the water in a turbulent cell far from equilibrium is convected upward by the heat below, it drags along most of the water molecules that compose it (Ilya Prigogine's example)

When a ribosome assembles 330 amino acids in four symmetric polypeptide chains (globins). Each traps an iron atom in a heme group at the center to form the hemoglobin protein, this is downward causal control of the amino acids, the heme groups, and the iron atoms

When 200 million of the 25 trillion red blood cells die each second, the 100 million hemoglobins in each cell must be replaced immediately. With the order of a few thousand bytes of information in each hemoglobin, this is 10 thousand x 100 million x 200 million = 2 x 1020 bits of information per second, a million times more information processing than today's fastest computer CPU.

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"We must admit that the mind of each one of our greatest geniuses — Aristotle, Kant or Leonardo, Goethe or Beethoven, Dante or Shakespeare — even at the moment of its highest flights of thought or in the most profound inner workings of the soul, was subject to the causal fiat and was a instrument in the hands of an almighty law which governs the world." Max Planck, Where Is Science Going, p.156.

[In Existentialism, the will condemns all the unchosen alternatives to nothingness as it grants being to the one chosen.]